1. Field of the Invention
The present invention relates to a vacuum-operated booster which has a movable diaphragm dividing the interior of a housing into a constant-pressure chamber and a variable-pressure chamber and which boosts an input by means of atmospheric pressure difference between the constant-pressure chamber and the variable-pressure chamber and outputs the boosted force.
2. Description of the Related Art
A vacuum-operated booster of the above-described type is disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. H10-230840. In the disclosed vacuum-operated booster, a valve body, which is connected to the movable diaphragm dividing the interior of the housing into a constant-pressure chamber and a variable-pressure chamber, has an axial hole, and a valve mechanism is built in the axial hole. The valve mechanism includes, as constituent elements, an input shaft, which is axially movable with respect to the valve body, a plunger connected to a front end portion of the input shaft, and a valve seat provided at the rear end of the plunger. Further, a reaction member, with which a front end portion of the plunger is engageable and which is axially movable with respect to the valve body, and an output shaft whose rear end portion is in engagement with the front face of the reaction member and which is axially movable with respect to the valve body, are assembled into the axial hole. When an input is applied to the input shaft, an output is generated at the output shaft, and a reaction force of the output is transmitted from the rear face of the reaction member to the valve body and the plunger via a reaction force dividing mechanism.
In the vacuum-operated booster disclosed in the above-mentioned publication, the valve body has a portion (pressure receiving area A1) which receives the reaction force directly from a radially outer portion of the rear face of the reaction member; and the plunger has a portion (pressure receiving area A2) which receives the reaction force directly from a radially inner portion of the rear face of the reaction member and a portion (pressure receiving area A3) which receives the reaction force from a radially intermediate portion of the rear face of the reaction member via a reaction force transmission/cutoff mechanism. The reaction force dividing mechanism, which transmits the reaction force of the output from the rear face of the reaction member to the valve body and the plunger, divides the reaction force from the reaction member into three forces (at a ratio corresponding to the pressure receiving area ratio A1:A2:A3).
The reaction force transmission/cutoff mechanism includes a sleeve, a spring seat, and a compression spring. The sleeve is fitted into a front shaft portion of the plunger such that the sleeve can telescopically move in the axial direction. A front end portion of the sleeve is engageable with the intermediate portion of the rear face of the reaction member, and the rear end of the sleeve is engageable, at its outer peripheral edge portion, with a stepped portion of the valve body. The spring seat is fixed to the plunger to be located rearward of the sleeve. The compression spring is assembled to the outer periphery of the plunger to be held between the spring seat and the sleeve with a predetermined load applied thereto.
Therefore, in the vacuum-operated booster disclosed in the above-mentioned publication, irrespective of the magnitude of the input, the valve body always receives reaction force directly from the reaction member at a predetermined division ratio (A1/(A1+A2+A3)). Further, during a low-input operation, which takes place until the sleeve moves rearward against the load of the compression spring and comes into engagement with the stepped portion of the valve body due to the reaction force that the sleeve receives from the reaction member, the plunger receives the reaction force directly from the reaction member and also receives the reaction force from the reaction member via the reaction force transmission/cutoff mechanism. However, during a high-input operation, which takes places after the sleeve comes into engagement with the stepped portion of the valve body, the reaction force transmitted from the reaction member via the reaction force transmission/cutoff mechanism is transmitted to the stepped portion of the valve body, so that the plunger becomes unable to receive the reaction force from the reaction member via the reaction force transmission/cutoff mechanism despite the plunger receiving the reaction force directly from the reaction member. Accordingly, in the vacuum-operated booster, the servo ratio (output/input) at the time of low-input operation becomes a low servo ratio ((A1+A2+A3)/(A2+A3)), and the servo ratio at the time of high-input operation becomes a high servo ratio ((A1+A2+A3)/A2).
Incidentally, since the vacuum-operated booster disclosed in the above-mentioned publication employs a structure in which the valve body always receives reaction force directly from the reaction member at a predetermined division ratio (A1/(A1+A2+A3)), the servo ratio at the time of low-input operation (low-load serve region) cannot be lowered to the vicinity of 1. Therefore, in the case where a vehicle is equipped with the above-described vacuum-operated booster and employs a brake system which increases electric power regeneration efficiency, increasing the electric power regeneration efficiency to a sufficient level is difficult.
Moreover, in the vacuum-operated booster disclosed in the above-mentioned publication, when the sleeve of the reaction force transmission/cutoff mechanism moves rearward against the load of the compression spring, an intermediate portion of the compression spring may be caught by (come into engagement with) the spring seat fixed to the plunger or the plunger itself, which may result in generation of noise and hinder smooth operation. Further, since the spring seat, the compression spring, and the sleeve of the reaction force transmission/cutoff mechanism must be individually assembled to the plunger, ease of assembly is poor.